1. Field of the Invention
The present invention relates to a cascode connection circuit used mainly in a high frequency band of 800 MHz or more.
2. Background Art
Cascode connection circuit having two field effect transistors (hereinafter referred to as a “FET”) cascode-connected to each other is used in a high frequency amplifier circuit used mainly in a high frequency band of 800 MHz or more.
FIG. 14 is a circuit diagram showing a conventional cascode connection circuit. As shown in FIG. 14, the conventional cascode connection circuit comprises FET1 having a source terminal connected to the ground, FET2 having a source terminal connected to a drain terminal of FET1, a capacitor 3 connected between the source terminal of FET1 and a gate terminal of FET2, a resistor 4 connected between a drain terminal of FET2 and the gate terminal of FET2, and a resistor 5 connected in parallel to the capacitor 3, between the source terminal of FET1 and the gate terminal of FET2 (refer to Japanese Patent Laid-Open No. 2005-33650, for example).
However, when capacitance C1 of the capacitor 3 is determined so that drain/source voltage Vds of FET1 and that of FET2 have the same phase and amplitude of alternate current, then a frequency region having a negative resistance emerges in the output side impedance of the cascode connection circuit. Thus, an oscillation tends to occur, causing a problem.